The present invention relates generally to laser hardened materials and structures, and more particularly to a novel optical filter structure for selectively blocking laser radiation of predetermined wavelength while passing radiation of other wavelengths.
Optical switching devices comprising transition or switching materials which are optically transparent in one state but which transform to an opaque metallic state when heated to a characteristic transition temperature are well developed for applications such as optical filters, modulators, laser output couplers, and the like. These devices are generally characterized by transition from substantial transparency below the characteristic transition temperature to substantial opacity above that temperature at all wavelengths.
In accordance with the teachings of copending application Ser. No. 07/172,119, which teachings are incorporated herein by reference, an optical filter structure was described for selectively blocking radiation of predetermined wavelength comprising a layer of material characterized by transition from non-metal to metal, corresponding to a change from transmissive to reflective states upon being heated to a characteristic transition temperature, deposited on a slab of dielectric material in the form of a plurality of multipole elements in an array of predetermined spacing between adjacent rows and columns of the array. The filter of the Ser. No. 07/172,119 application is characterized by substantial transparency to all radiation wavelengths below its characteristic transition temperature, and opacity only to the predetermined wavelength, and substantial transparency to all other wavelengths, above the transition temperature, and is constructed by applying a film of thermal switching material in the form of an array of microwave-type multipole elements of optical dimensions, the film being applied to a dielectric slab, or sandwiched between two dielectric slabs.
For purposes of describing the invention and defining the scope thereof, the term "optical" shall, in accordance with the customary usage, be defined herein to include only ultraviolet, visible, near infrared, mid-infrared and far infrared regions of the electromagnetic spectrum lying between about 0.1 to about 1000 microns (see e.g. Optical Physics, by Max Garbuny, Academic press, New York, 1965, pp 1-6). and more specifically to the range of from about 0.2 micron, the approximate lower limit of operation of fine quality quartz lenses (Garbuny, p 280), to about 50 microns, the approximate upper limit of operation of long wavelength transmitting material such as KRS-5 (thallium bromide-iodide ionic crystal) (Garbuny, p 282).
The invention provides a novel optical reflection band filter which switches from a reflective state to a transmissive state upon being heated to a transition temperature following absorption of a certain amount of laser radiation. The filter is constructed by applying films of high-temperature superconducting materials in the form of an array of microwave type multipole elements of optical dimensions applied to a dielectric slab or sandwiched between two dielectric slabs.
The switching materials operate by undergoing a superconducting-to-insulating transition at the characteristic temperature. The invention therefore is a substantial improvement over the totally opaque switching reflection filter, the non-switching metallic element array reflection-band filter, and other wavelength responsive laser hardening devices.
It is therefore a principal object of the invention to provide an improved optical filter.
It is a further object of the invention to provide an optical filter for selectively blocking laser radiation of selected wavelength while passing radiation of substantially all other wavelengths.
It is yet a further object of the invention to provide a switchable filter incorporating insulating-to-superconducting materials to provide selective wavelength filtering.
It is yet another object of the invention to provide an optical filter having fast response time.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.